Close duplicate editors on same tabbar

[colin-grant-work]

What it does

Implements behavior similar to VSCode's where editors already on a given tabbar for a given URI are closed when a new editor for that URI is moved onto the tabbar.

How to test

1. Split an editor.
2. Change the states of the two editors.
3. Move one editor onto the other's tabbar.
4. Observe that the moved editor remains open, and the other editor is closed.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[msujew]

Looks good to me 👍

• Moving a duplicated editor from one group to another that already has an editor for that file closes the editor correctly
• Other information in the model, such as dirty state, etc. are kept as expected.

[vince-fugnitto]

I second Mark's comments and confirm the changes work as expected 👍